Valence Electrons Everything that is underlined should get filled in on your notes! Valence Electrons • Valence electrons • the electrons that are in the highest (outermost) energy level • that level is also called the valence shell of the atom • they are held most loosely — The number of valence electrons in an atom determines: — The proper;es of the atom — The way that atom will bond chemically — As a rule, the fewer electrons in the valence shell, the more reac;ve the element is. — When an atom has eight electrons in the valence shell, it is stable. • Atoms usually react in a way that makes each atom more stable. • There are two ways this can happen: • The number of valence electrons increases to eight • Loosely held valence electrons are given up Chemical Bonds • A chemical bond is the force of attraction that holds two atoms together as a result of the rearrangement of electrons between them. • There are two types of chemical bonds: • Ionic • Covalent Valence Electrons by Group Group # Group Name # of valence electrons 1 Alkali Metals 1 2 Alkaline Earth Metals 2 3-­‐12 Transi;on Metals 1 or 2 13 Boron Group 3 14 Carbon Group 4 15 Nitrogen Group 5 16 Oxygen Group 6 17 Halogens 7 18 Noble Gases 8 Please copy this chart in your notes!! Patterns on the Periodic Table — The number of valence electrons increases as you go across the periodic table. — When you start each new period, the number of valence electrons drops down to one and begins increasing. Drawing Atoms • When scientists draw atoms to show how they chemically bond, they only draw the valence electrons. • We only use the valence electrons because those are the only ones involved in chemical bonding. Electron Dot Diagrams — Electron Dot Diagrams are drawings that include the chemical symbol for the element surrounded by dots. — Each dot stands for one valence electron. — These are also some;mes called Lewis Dot Diagrams. — All atoms want a full valence shell (because that is how they become stable!) — So, they will share or transfer electrons with other atoms to achieve stability. — Nega;ve ions form when atoms gain electrons. — Posi;ve ions form when atoms lose electrons. Steps for drawing an Electron Dot Diagram
1. Write the element’s chemical symbol
2. Look on the periodic table to see what group
the element is in
3.  Use the chart in your notes to determine how
many valence electrons the element has
4. Draw the dots around the chemical symbol
starting at the top and moving clockwise
around the symbol
Examples (copy these in your notes!) Try these on your own: Electron
Configuration:
Ions and Excite State
Electron Configuration - Cations
• Cations – atoms that lose electrons - metals
• Electron Configuration:
• Magnesium – 12 electrons in its neutral state
• 1s22s22p63s2
• Magnesium ion
• Loses 2 electrons
• New configuration: 1s22s22p6
• Notice that the electron configuration is
the same as the noble gas Neon
Electron Configuration - Anions
•  Anions – gain electrons – non-metals
•  Electron Configuration:
•  Chlorine – 17 electrons in its neutral state
•  1s22s22p63s23p5
•  Chlorine ion
•  Gains 1 electron
•  New configuration: 1s22s22p63s23p6
•  Notice that the electron configuration is the same
as the noble gas Argon
•  This indicates that by gaining 1 electron, it
obtains an outer octet (stable valence
configuration!)
Electron Configuration
• Both ions obtain a noble gas configuration
• This makes the ion and the noble gas
isoelectronic
• Isoelectronic: when two elements and/or
ions have the same electronic
configurations with one another
• They tend to have similar chemical
properties.
Isoelectric Examples
•  Li+1
•  He
1s2
1s2
•  S-2
•  Ar
1s2 2s2 2p6 3s2 3p6
1s2 2s2 2p6 3s2 3p6
Ionic configuration problems
Cations
Anions
•  Determine the electron
configuration for:
•  Aluminum - Al+3
•  Determine the electron
configuration for:
•  Nitrogen – N-3
•  Calcium – Ca+2
•  Fluorine – F-1
•  Potassium - K+1
•  Oxygen - O-2
Excited State Electrons
•  When an electron absorbs energy, it jumps to a higher energy
level, then drop back down giving off energy in the form of
light and heat
Energy
Energy in the form of light
Excited State Electrons
•  The electron configuration for an excited state electron
shows one or more electrons in a higher energy level
Excited State Electrons
•  You need to be able to determine an electron in the excited
state
•  Ex:
Element
Fluorine
Sodium
Ground State
1s2 2s2 2p5
1s2 2s2 2p6 3s1
Excited State
1s2 2s22p 4 3s1
1s2 2s2 2p6 3p1